Dual fuel refueling

ABSTRACT

Embodiments are disclosed that relate to refueling a dual fuel internal combustion engine. In one example, a method comprises supplying a liquid fuel to a fuel tank configured to store both the liquid fuel and a gaseous fuel, if a pressure in the fuel tank is less than a threshold pressure.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 14/275,596, entitled “DUAL FUEL REFUELING,” filed on May 12, 2014.The entire contents of the above-referenced application are herebyincorporated by reference in its entirety for all purposes.

FIELD

The field of the disclosure relates to dual fuel internal combustionengines.

BACKGROUND AND SUMMARY

Compressed natural gas (CNG) is a high octane fuel that is beneficialfor reducing engine knock, hydrocarbon emissions in cold start events,and carbon dioxide emissions during engine operation. However, CNG has alow energy density compared to liquid hydrocarbon fuels, such as dieselfuel or gasoline. To increase the range and total fuel quantity storedin a vehicle, CNG may be utilized in conjunction with gasoline or dieselfuel, requiring the vehicle to switch between fuels for optimalperformance. To facilitate the consumption of both gaseous and liquidfuels, two separate fuel tanks may be included in the vehicle to storethe gaseous and liquid fuels, respectively.

The inventors herein have recognized several issues with the aboveapproach. Namely, the use of two separate fuel tanks that respectivelystore gaseous and liquid fuels increases vehicle weight, packagingspace, complexity of fuel storage and injection, and reduces fueleconomy. These issues are exacerbated for light duty vehicles.

One approach that at least partially addresses the above issues includesa method of refueling comprising supplying a liquid fuel to a fuel tankconfigured to store both the liquid fuel and a gaseous fuel, if apressure in the fuel tank is less than a threshold pressure.

In a more specific example, liquid fuel is supplied to the fuel tank ifthe liquid fuel is sensed in a surge tank configured to store the liquidfuel, the surge tank fluidly coupled to the fuel tank and positionedupstream of the fuel tank.

In another aspect of the example, vapor from the fuel tank is pumped toa secondary tank if the pressure in the fuel tank is greater than thethreshold pressure.

In this way, a vehicle may be supplied and refueled with two differentfuels by using a single fuel tank to store the fuels, reducing vehicleweight, packaging space, complexity of fuel storage and injection, andincreasing fuel economy. Thus, the technical result is achieved by theseactions.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an example cylinder of an internalcombustion engine.

FIG. 2 shows a schematic depiction of the engine of FIG. 1 and a fuelsystem configured to operate on a mix of gaseous fuel and liquid fuel.

FIG. 3 shows a schematic depiction of the engine of FIG. 1 and anotherfuel system configured to operate on a mix of gaseous fuel and liquidfuel.

FIG. 4 shows a flowchart illustrating an exemplary method for operatingthe engine and fuel system of FIGS. 1-3.

FIG. 5 shows a flowchart illustrating an exemplary method for refuelingthe fuel system of FIG. 2.

FIG. 6 shows a flowchart illustrating an exemplary method for refuelingthe fuel system of FIG. 3.

DETAILED DESCRIPTION

A key physical property of natural gas (e.g., methane) is its solubilityin hydrocarbons. This property can be utilized to store both liquidhydrocarbon fuels (e.g., gasoline, diesel, etc.) and compressed naturalgas (CNG) in a single fuel tank. By varying the pressure of the fuels,the admixture may be separated into its constituent gaseous and liquidcomponents for discrete consumption. The present description relates tosystems and methods for refueling such a fuel tank. FIG. 1 schematicallydepicts an example cylinder of an internal combustion engine, FIG. 2shows a schematic depiction of the engine of FIG. 1 and a fuel systemconfigured to operate on a mix of gaseous fuel and liquid fuel, FIG. 3shows a schematic depiction of the engine of FIG. 1 and another fuelsystem configured to operate on a mix of gaseous fuel and liquid fuel,FIG. 4 shows a flowchart illustrating an exemplary method for operatingthe engine and fuel system of FIGS. 1-3, FIG. 5 shows a flowchartillustrating an exemplary method for refueling the fuel system of FIG.2, and FIG. 6 shows a flowchart illustrating an exemplary method forrefueling the fuel system of FIG. 3. The engine of FIG. 1 also includesa controller to carry out the methods depicted in FIGS. 4-5.

FIG. 1 depicts an example embodiment of a combustion chamber or cylinderof internal combustion engine 10. Engine 10 may be controlled at leastpartially by a control system 13, including controller 12, and by inputfrom a vehicle operator 130 via an input device 132. In one example,input device 132 includes an accelerator pedal and a pedal positionsensor 134 for generating a proportional pedal position signal PP.Cylinder (e.g., combustion chamber) 14 of engine 10 may includecombustion chamber walls 136 with piston 138 positioned therein. Piston138 may be coupled to crankshaft 140 so that reciprocating motion of thepiston is translated into rotational motion of the crankshaft.Crankshaft 140 may be coupled to at least one drive wheel of thepassenger vehicle via a transmission system. Further, a starter motormay be coupled to crankshaft 140 via a flywheel to enable a startingoperation of engine 10.

Cylinder 14 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 can communicate with othercylinders of engine 10 in addition to cylinder 14. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 1 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor or the engine. A throttle 162 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 162 may be disposed downstreamof compressor 174 as shown in FIG. 1, or may alternatively be providedupstream of compressor 174.

Exhaust passage 148 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178.Sensor 128 may be any suitable sensor for providing an indication ofexhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO(universal or wide-range exhaust gas oxygen), a two-state oxygen sensoror EGO (as depicted), a HEGO (heated EGO), a NOx, HC, or CO sensor.Emission control device 178 may be a three way catalyst (TWC), NOx trap,various other emission control devices, or combinations thereof.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 14. In some embodiments, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 via actuator 152.Similarly, exhaust valve 156 may be controlled by controller 12 viaactuator 154. During some conditions, controller 12 may vary the signalsprovided to actuators 152 and 154 to control the opening and closing ofthe respective intake and exhaust valves. The position of intake valve150 and exhaust valve 156 may be determined by respective valve positionsensors (not shown). The valve actuators may be of the electric valveactuation type or cam actuation type, or a combination thereof. Theintake and exhaust valve timing may be controlled concurrently or any ofa possibility of variable intake cam timing, variable exhaust camtiming, dual independent variable cam timing or fixed cam timing may beused. Each cam actuation system may include one or more cams and mayutilize one or more of cam profile switching (CPS), variable cam timing(VCT), variable valve timing (VVT) and/or variable valve lift (VVL)systems that may be operated by controller 12 to vary valve operation.For example, cylinder 14 may alternatively include an intake valvecontrolled via electric valve actuation and an exhaust valve controlledvia cam actuation including CPS and/or VCT. In other embodiments, theintake and exhaust valves may be controlled by a common valve actuatoror actuation system, or a variable valve timing actuator or actuationsystem.

Cylinder 14 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen for example when higher octane fuels or fuelswith higher latent enthalpy of vaporization are used. The compressionratio may also be increased if direct injection is used due to itseffect on engine knock.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 14 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more fuel injectors for providing fuel thereto. As a non-limitingexample, cylinder 14 is shown including two fuel injectors 166 and 170.Fuel injector 166 is shown coupled directly to cylinder 14 for injectingfuel directly therein in proportion to the pulse width of signal FPW-1received from controller 12 via electronic driver 168. In this manner,fuel injector 166 provides what is known as direct injection (hereafterreferred to as “DI”) of fuel into combustion cylinder 14. While FIG. 1shows injector 166 as a side injector, it may also be located overheadof the piston, such as near the position of spark plug 192. Such aposition may aid in mixing and combustion when operating the engine withan alcohol-based fuel due to the lower volatility of some alcohol-basedfuels. Alternatively, the injector may be located overhead and near theintake valve to aid in mixing of intake air and injected fuel. Fuel maybe delivered to fuel injector 166 from fuel system 172 including a fueltank, fuel pumps, a fuel rail, and driver 168. Alternatively, fuel maybe delivered by a single stage fuel pump at lower pressure, in whichcase the timing of the direct fuel injection may be more limited duringthe compression stroke than if a high pressure fuel system is used.Further, although not shown in FIG. 1, the fuel tank may have a pressuretransducer providing a signal to controller 12.

Fuel injector 170 is shown arranged in intake passage 146, rather thanin cylinder 14, in a configuration that provides what is known as portinjection of fuel (hereafter referred to as “PFI”) into the air intakeport upstream of cylinder 14. Fuel injector 170 may inject fuel inproportion to the pulse width of signal FPW-2 received from controller12 via electronic driver 171. Fuel may be delivered to fuel injector 170by fuel system 172.

Fuel may be delivered by both injectors to the cylinder during a singlecycle of the cylinder. For example, each injector may deliver a portionof a total fuel injection that is combusted in cylinder 14. Further, thedistribution and/or relative amount of fuel delivered from each injectormay vary with operating conditions such as described herein below. Therelative distribution of the total injected fuel among injectors 166 and170 may be referred to as a first injection ratio. For example,injecting a larger amount of the fuel for a combustion event via (port)injector 170 may be an example of a higher first ratio of port to directinjection, while injecting a larger amount of the fuel for a combustionevent via (direct) injector 166 may be a lower first ratio of port todirect injection. Note that these are merely examples of differentinjection ratios, and various other injection ratios may be used.Additionally, it should be appreciated that port injected fuel may bedelivered during an open intake valve event, closed intake valve event(e.g., substantially before an intake stroke, such as during an exhauststroke), as well as during both open and closed intake valve operation.Similarly, directly injected fuel may be delivered during an intakestroke, as well as partly during a previous exhaust stroke, during theintake stroke, and partly during the compression stroke, for example.Further, the direct injected fuel may be delivered as a single injectionor multiple injections. These may include multiple injections during thecompression stroke, multiple injections during the intake stroke or acombination of some direct injections during the compression stroke andsome during the intake stroke. When multiple direct injections areperformed, the relative distribution of the total directed injected fuelbetween an intake stroke (direct) injection and a compression stroke(direct) injection may be referred to as a second injection ratio. Forexample, injecting a larger amount of the direct injected fuel for acombustion event during an intake stroke may be an example of a highersecond ratio of intake stroke direct injection, while injecting a largeramount of the fuel for a combustion event during a compression strokemay be an example of a lower second ratio of intake stroke directinjection. Note that these are merely examples of different injectionratios, and various other injection ratios may be used. Furthermore theinjection ratios may be adjusted based on one or more engine operatingconditions such as engine load, engine speed, fuel system pressure,engine temperature, and the like. In this way one or both of liquid andgaseous fuels may be combusted in an engine cylinder.

As such, even for a single combustion event, injected fuel may beinjected at different timings from a port and direct injector.Furthermore, for a single combustion event, multiple injections of thedelivered fuel may be performed per cycle. The multiple injections maybe performed during the compression stroke, intake stroke, or anyappropriate combination thereof.

As described above, FIG. 1 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

Fuel injectors 166 and 170 may have different characteristics. Theseinclude differences in size, for example, one injector may have a largerinjection hole than the other. Other differences include, but are notlimited to, different spray angles, different operating temperatures,different targeting, different injection timing, different spraycharacteristics, different locations etc. Moreover, depending on thedistribution ratio of injected fuel among injectors 170 and 166,different effects may be achieved. Further still, fuel injectors 166 and170 may each include one or more gaseous fuel injectors for injectinggaseous fuel, and one or more liquid fuel injectors for injecting liquidfuel.

In some embodiments, fuel system 172 may include a fuel tank that holdsa liquid fuel, such as gasoline, and also holds a gaseous fuel, such asCNG. A portion of the gaseous fuel may be solubilized in the liquidfuel. The liquid fuel and the gaseous fuel together may be referred toas a mixed fuel, and the fuel tank may thus store or hold a mixed fuel.In other embodiments, fuel system 172 may include one fuel tank ormultiple fuel tanks. In embodiments where fuel system 172 includesmultiple fuel tanks, the fuel tanks may hold fuel with the same fuelqualities or may hold fuel with different fuel qualities, such asdifferent fuel compositions. These differences may include differentalcohol content, different octane, different heat of vaporizations,different fuel blends, and/or combinations thereof etc. In one example,fuels with different alcohol contents could include gasoline, ethanol,methanol, or alcohol blends such as E85 (which is approximately 85%ethanol and 15% gasoline) or M85 (which is approximately 85% methanoland 15% gasoline). Other alcohol containing fuels could be a mixture ofalcohol and water, a mixture of alcohol, water and gasoline etc. Fuelinjectors 166 and 170 may be configured to inject fuel from the samefuel tank, from different fuel tanks, from a plurality of the same fueltanks, or from an overlapping set of fuel tanks. While FIG. 1 depictsfuel injector 166 as a direct fuel injector and fuel injector 170 as aport fuel injector, in other embodiments both injectors 166 and 170 maybe configured as port fuel injectors or may both be configured as directfuel injectors.

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 110 in this particular example, random access memory 112,keep alive memory 114, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type)coupled to crankshaft 140; throttle position (TP) from a throttleposition sensor; an exhaust gas oxygen (EGO) signal from exhaust gassensor 128; and absolute manifold pressure signal (MAP) from sensor 124.Engine speed signal, RPM, may be generated by controller 12 from signalPIP. Manifold pressure signal MAP from a manifold pressure sensor may beused to provide an indication of vacuum, or pressure, in the intakemanifold.

Storage medium read-only memory 110 can be programmed with computerreadable data representing instructions executable by processor 106 forperforming the methods described below as well as other variants thatare anticipated but not specifically listed. Example routines that maybe performed by the controller are described herein and with regards toFIGS. 4 and 5.

Turning now to FIG. 2, a schematic depiction of engine 10 of FIG. 1 andfuel system 172 configured to operate on a mix of gaseous fuel andliquid fuel is shown. As shown, internal combustion engine 10 includescylinders 14 coupled to intake passage 144 and exhaust passage 148.Intake passage 144 may include throttle 162. Exhaust passage 148 mayinclude emissions control device 178. Control system 13, includingcontroller 12, may receive signals from various sensors 16, andadditional sensors shown in FIGS. 1 and 2, and output signals to variousactuators 81, including additional actuators shown in FIGS. 1 and 2.

Cylinders 14 may be configured as part of cylinder head 201. In FIG. 2,cylinder head 201 is shown with 4 cylinders in an inline configuration.In some examples, cylinder head 201 may have more or fewer cylinders,for example six cylinders. In some examples, the cylinders may bearranged in a V configuration or other suitable configuration.

Cylinder head 201 is shown coupled to fuel system 172. Cylinder 14 isshown coupled to fuel injectors 166A and 166B, and fuel injectors 170Aand 170B. Although only one cylinder is shown coupled to fuel injectors,it is to be understood that all cylinders 14 included in cylinder head201 may also be coupled to one or more fuel injectors. In this exampleembodiment, fuel injectors 166A and 166B are depicted as a direct fuelinjector and fuel injectors 170A and 170B are depicted as a port fuelinjector. Although only two direct injectors and two port injectors areshown in FIG. 2, it is to be understood that engine 10 may comprise morethan two direct injectors and more than two fuel injectors. Each fuelinjector may be configured to deliver a specific quantity of gaseousand/or liquid fuel at a specific time point in the engine cycle inresponse to commands from controller 12. In some examples, fuelinjectors 170A and 170B may be used to inject a gaseous fuel such asCNG, while in other examples fuel injectors 166A and 166B may be used toinject gaseous fuel. In the latter example in which fuel injectors 166Aand 166B are used to inject gaseous fuel, liquid fuel such as gasolineor diesel may be injected via fuel injectors 166A and 166B as well,though embodiments in which fuel injectors 170A and 170B are used toinject liquid fuel in combination with direct injection of the gaseousfuel are also contemplated. One or more fuel injectors may be utilizedto deliver combustible fuel to cylinder 14 during each combustion cycle.The timing and quantity of fuel injection may be controlled as afunction of engine operating conditions.

Fuel system 172 includes fuel tank 200. Fuel tank 200 may include aliquid fuel, such as gasoline, diesel fuel, or a gasoline-alcohol blend(e.g. E10, E85, M15, or M85), and may also include a gaseous fuel, suchas CNG. Fuel tank 200 may be configured to store liquid fuel and gaseousfuel together at a relatively low pressure compared to conventional CNGstorage (e.g. 200-250 atmospheres). For example, the gaseous fuel may beadded to a pressure of 100 atmospheres. In this way, a portion of thegaseous fuel may be dissolved in the liquid fuel. At 100 atmospheres,CNG may dissolve in gasoline to the point where 40% of the liquid fuelcomponent in fuel tank 200 is CNG. Fuel tank 200 may include pressuresensor 211, temperature sensor 212, and liquid level sensor 215 (e.g., afloat sensor).

Liquid fuel and/or gaseous fuel may be supplied from fuel tank 200 tocylinders 14 of engine 10 via liquid fuel line 220 and gaseous fuel lineand 221, fuel rails 205 and 206 and fuel injectors 166A, 166B, 170A, and170B. In one example, gaseous fuel may be delivered from fuel tank 200by positioning three-way gaseous fuel switching valve 224 to fluidlycouple fuel tank 200 to gaseous fuel line 221 and gaseous fuel rail 205.Gaseous fuel delivered to gaseous fuel rail 205 may be port fuelinjected to cylinder 14 by gaseous fuel injector 170B, and may bedirectly injected to cylinder 14 by liquid fuel injector 170A. Liquidfuel, including solubilized gaseous fuel in the liquid fuel, may besupplied from fuel tank 200 by operating fuel lift pump 210. Liquid fuelline 220 may be coupled to a lower portion of fuel tank 200 in orderdraw liquid fuel from fuel tank 200 via fuel lift pump 210. In somecases, fuel lift pump 210 may be omitted from fuel system 172. In suchembodiments, the pressure of gaseous fuel stored in fuel tank 200 may beused to drive liquid fuel from fuel tank 200 to fuel rail 205 via fuelline 220. In embodiments where fuel lift pump 210 is omitted, anadditional liquid fuel valve may be coupled to fuel line 220 to controlliquid fuel flow through fuel line 220. If fuel separator bypass valve226 is open, liquid fuel may be delivered via bypass fuel line 228 toliquid fuel line 220 and liquid fuel rail 206, where liquid fuel may bedirectly injected into cylinder 14 via liquid fuel injector 166A and/orport fuel injected into cylinder 14 liquid fuel injector 166B.

In one example, gaseous fuel rail 205 may comprise a DI gaseous fuelrail for direct injecting gaseous fuel via one or more DI gaseous fuelinjectors 170A and a PFI gaseous fuel rail for port injection of gaseousfuel via one or more PFI liquid fuel injectors 170B. In other examples,only a DI gaseous or only a PFI gaseous injection system may be used.Furthermore, liquid fuel rail 206 may comprise a DI liquid fuel rail fordirect injecting liquid fuel via one or more DI liquid fuel injectors166A and a PFI liquid fuel rail for port injection of liquid fuel viaone or more PFI liquid fuel injectors 166B. In other examples, only a DIliquid or only a PFI liquid injection system may be used. Further still,a DI gaseous fuel pump may be provided upstream of DI gaseous fuel railand downstream of gaseous fuel switching valve 224 for deliveringpressurized gaseous fuel to DI gaseous fuel rail. Further still, a DIliquid fuel pump may be provided upstream of DI liquid fuel rail anddownstream of bypass fuel line 228 for delivering pressurized liquidfuel to DI liquid fuel rail. Further still, a single DI fuel pump may beused to deliver both gaseous fuel and liquid fuel. Although not shown inFIG. 2, DI liquid fuel pump may be a high pressure fuel pump comprisinga solenoid activated inlet check valve, a piston, and an outlet checkvalve for delivering high pressure liquid fuel to DI liquid fuel rail.Injection of liquid fuel via DI liquid fuel injection pump may lubricatethe piston of liquid DI fuel pump, thereby reducing pump wear anddegradation and reducing pump NVH.

If fuel separator bypass valve 226 is closed, liquid fuel supplied fromfuel tank 200 may be delivered to fuel separator 230. As an example,fuel separator 230 may comprise a coalescer or other known processingunit for separating liquids and gases. Fuel separator 230 may comprise acoalescing filter 234, and an expansion chamber 232 on a downstream sidethereof and a sump chamber 236 on an upstream side thereof. Liquid fuelmay be supplied to the sump chamber 236 and/or the coalescing filter 234at a fuel pressure of fuel lift pump 210. A pressure differential may bemaintained across coalescing filter 234 wherein a pressure in expansionchamber 232 may be less than the fuel pressure in the sump chamber 236.For example, the pressure differential may be maintained by controllinglift pump 210 to supply sufficient pressure. Furthermore, coalescingfilter may comprise a fritted filter, such as a steel fritted filter.Expansion chamber 232 may also be described as a manifold chamber.

In one example, the pressure differential may be maintained acrosscoalescing filter 234 by a check valve 242 positioned downstream of thefuel separator 230 and fluidly coupled to the expansion chamber 232 viaa gaseous fuel relief passage 240. The check valve 242 may be configuredto open when a pressure upstream of the check valve exceeds a thresholdpressure, for example, an intake manifold pressure. The outlet of thecheck valve 242, via the gaseous fuel relief passage 240, may be fluidlycoupled to the intake manifold and/or a positive crankcase ventilation(PCV) system of engine 10. In this way, separated gaseous fuel may besupplied to the intake manifold and/or the engine crankcase where it maybe used to aid in reducing oil viscosity and in lubricating the enginecomponents.

For example, a pressure in the expansion chamber 232 may be less than athreshold pressure. The pressure in expansion chamber 232 may bemeasured by pressure sensor 238 and communicated to controller 12. Inone example, the threshold pressure may comprise a pressure less than100 psi. Below 100 psi, the solubility of CNG, methane, and othergaseous fuels may be reduced such that an amount of solubilized gaseousfuel in the liquid fuel may be very low. For example, the solubility ofgaseous fuel (volume gaseous fuel dissolved per volume of liquid fuel)may be approximately 1 mL/mL per atmospheres of gaseous fuel pressure.Accordingly, upon entering the fuel separator 230, gaseous fuelsolubilized in the liquid fuel may be desolubilized and volatilized fromthe liquid fuel, and may be conveyed across the coalescing filter 234into the expansion chamber 232 and out of fuel separator 230 towardsgaseous fuel switching valve 224. Furthermore, positioning the three-waygaseous fuel switching valve 224 to fluidly connect the fuel separator230 to gaseous fuel rail 205 may supply the desolubilized gaseous fuelto gaseous fuel rail 205. Subsequently, desolubilized gaseous fuel maybe injected to cylinder 14 via gaseous fuel injectors 170A and 170B.

Volatilization of the gaseous fuel from the liquid fuel may decrease theliquid and gaseous fuel temperatures and may cool the coalescing filter234. Furthermore, the lowered liquid fuel temperature may reduce thevolatility of the liquid fuel so that entrainment of lighter hydrocarbonfuel components into the desolubilized gaseous fuel stream may bereduced. Any entrained hydrocarbon components in the liquid fuel, suchas residual butanes, pentanes, and hexanes, may volatilize and beentrained by the desolubilized gaseous fuel in the fuel separator.Accordingly, the octane value of the liquid fuel may thus be slightlyraised, while the octane value of the gaseous fuel may be slightlyreduced. Furthermore, recompression of the gaseous fuel downstream ofthe fuel separator may condense these residual hydrocarbon components.

Liquid fuel delivered to the fuel separator 230 may flow through sumpchamber 236 and out of fuel separator 230 towards bypass fuel line 228and liquid fuel line 220. A portion of liquid fuel delivered to fuelseparator 230 may condense as droplets on and within coalescing filter234. As the droplets of liquid fuel flow through the coalescing filter234, they may merge and coalesce, thereby forming larger droplets, whichmay be conveyed by gravity back to the sump chamber 236.

Although in FIG. 2, the sump chamber 236, coalescing filter 234, andexpansion chamber 232 are illustrated as being arranged in a linearfashion, other arrangements may also be included. For example, the sumpchamber 236, coalescing filter 234, and expansion chamber 232 may bearranged in a concentric configuration wherein the expansion chamber isencircled by the coalescing filter 234 and the sump chamber 236, andwherein the fuel flowing into and out of the fuel separator 230 flows inan axial and/or radial direction with respect to the concentricconfiguration. Furthermore, sump chamber 236 may be fluidly connected toexpansion chamber 232 via coalescing filter 234.

In this way, solubilized gaseous fuel in the liquid fuel may bedesolubilized and separated from the liquid fuel before injection ofliquid fuel into cylinder 14. Furthermore, gaseous fuel may be injectedseparately from liquid fuel to cylinder 14 via gaseous fuel injectors170A and 170B. In other words, gaseous fuel may be injected only viagaseous fuel injectors and liquid fuel may be injected only via liquidfuel injectors. Furthermore, only gaseous fuel may be injected byswitching off liquid fuel injection, or only liquid fuel may be injectedby switching off gaseous fuel injection. Gaseous fuel may comprise oneor more of compressed natural gas (CNG), methane, propane, and butane asnon-limiting examples, while liquid fuel may comprise one or more ofgasoline, alcohol, and diesel fuel, as non-limiting examples.

Fuel system 172 is shown coupled to refueling system 250. Refuelingsystem 250 may be coupled to fuel tank 200 via tank access valve 218.Tank access valve 218 may be coupled to refueling conduit 260. Refuelingconduit 260 may include high pressure refueling port 255. High pressurerefueling port 255 may be configured to receive a pressurized gaseousfuel pump nozzle, or a fuel pump nozzle configured to deliver apre-pressured mixture of liquid fuel and gaseous fuel. In some cases, asecond high pressure refueling port may be included to allowcompatibility with more than one type of high pressure fuel pumpnozzle—for example, a first refueling port may be configured to receivea pressurized mixture of liquid fuel and gaseous fuel, and a secondrefueling port may be configured to receive the gaseous fuel.

Access to high pressure refueling port 255 may be regulated by refuelinglock 257. In some embodiments, refueling lock 257 may be a fuel caplocking mechanism. The fuel cap locking mechanism may be configured toautomatically lock a fuel cap in a closed position so that the fuel capcannot be opened. For example, the fuel cap may remain locked viarefueling lock 257 while pressure in the fuel tank is greater than athreshold. A fuel cap locking mechanism may be a latch or clutch, which,when engaged, prevents the removal of the fuel cap. The latch or clutchmay be electrically locked, for example, by a solenoid, or may bemechanically locked, for example, by a pressure diaphragm.

In some embodiments, refueling lock 257 may be a filler pipe valvelocated at a mouth of refueling conduit 260. In such embodiments,refueling lock 257 may prevent the insertion of a refueling pump intorefueling conduit 260. The filler pipe valve may be electrically locked,for example by a solenoid, or mechanically locked, for example by apressure diaphragm.

In some embodiments, refueling lock 257 may be a refueling door lock,such as a latch or a clutch which locks a refueling door located in abody panel of the vehicle. The refueling door lock may be electricallylocked, for example by a solenoid, or mechanically locked, for exampleby a pressure diaphragm.

In embodiments where refueling lock 257 is locked using an electricalmechanism, refueling lock 257 may be unlocked by commands fromcontroller 12. In embodiments where refueling lock 257 is locked using amechanical mechanism, refueling lock 257 may be unlocked via a pressuregradient.

Refueling conduit 260 may be coupled to low pressure refueling conduit280. Low pressure refueling conduit 280 may be coupled to surge tank270. Surge tank 270 may include a low pressure refueling port 265 and aliquid sensor 275. Low pressure refueling conduit 280 may include fuelpump 285 and check valve 290. Fuel pump 285 may only operate when fueltank pressure is below a threshold, and may only operate when there isliquid fuel in surge tank 270, as sensed by liquid sensor 275. In thisway, fuel pump 285 may not pump an air/fuel mixture into fuel tank 200.Further, when fuel tank pressure reaches a threshold, fuel pump 285 maybe shut off by controller 12, causing liquid fuel to accumulate in surgetank 270. This may cause a low pressure liquid fuel dispenser nozzleengaged with low pressure refueling port 265 to turn itself off. Accessto refueling port 265 may be regulated by refueling lock 267. Refuelinglock 267 may comprise one of the examples described for refueling lock257. Refueling locks 257 and 267 may further comprise differentmechanisms.

Fuel system 172 may allow unpressurized liquid fuel (e.g., gasoline,diesel, alcohol, etc.), pressurized gaseous fuel (e.g., CNG), and/or apressurized mixture of liquid and gaseous fuel (e.g., gasoline and CNG,diesel and CNG, etc.) to be added to fuel tank 200 as desired, as longas pressure in the fuel tank is less than a maximum allowable pressure.Addition of pressured fuel in particular does not require an activecontrol mechanism. When liquid is sensed inside surge tank 270, fuelpump 285 may be used to pump the liquid fuel into the tank if the tankpressure is less than the maximum allowable pressure. Liquid sensor 275may be used to avoid pumping air into tank 270 and to avoid creation ofa combustible mixture inside the tank. When tank 270 is full (e.g., withrespect to pressure or liquid level), fuel pump 285 stops, liquid fuelfills surge tank 270, and a liquid fuel dispenser nozzle may shut itselfoff. In some examples, control logic may be employed to reduceevaporative emissions from surge tank 270 by running fuel pump 285 at alater time. FIG. 5 shows a flowchart illustrating an exemplary method500 that may be used to refuel fuel tank 200 via fuel system 172.

As described above, in some embodiments fuel system 172 may include twohigh pressure refueling ports. In this example, a first high pressurerefueling port may be configured to receive a pre-pressurized mixture ofliquid fuel and gaseous fuel, and a second high pressure refueling portmay be configured to receive the gaseous fuel. The nozzle/port designutilized by the first high pressure refueling port may differ from thatutilized by the second high pressure refueling port so that the correctfuel(s) are supplied to the corresponding port, for example.

Turning now to FIG. 3, a schematic depiction of engine 10 of FIG. 1 anda fuel system 300 configured to operate on a mix of gaseous fuel andliquid fuel is shown. As fuel system 300 may be considered amodification of fuel system 172, like reference numerals are used whereappropriate, with their functionality being understood from thedescription above.

Unlike fuel system 172, fuel system 300 lacks a surge tank and fuel pumpat low pressure refueling port 265. As such, the addition of liquid fuelmay be carried out when the pressure in fuel tank 200 is approximatelyzero. To depressurize main tank 200, substantially all (e.g., 90% ormore) of the solubilized gaseous fuel in the main tank may be drawn toand consumed by engine 10. Alternatively or additionally, a mechanismmay be provided by which main tank 200 may be depressurized by ventingsolubilized gaseous fuel to the atmosphere or to a separate tank notshown in FIG. 3. In these embodiments, refueling may be performedwithout active control mechanism: if a substantially non-zero pressure(e.g., atmospheric or greater pressure) exists in main tank 200, checkvalve 290 may allow liquid fuel to fill low pressure refueling port 265and a low pressure liquid fuel dispenser nozzle may automatically shutitself off.

FIG. 3 also shows a secondary tank 304 and a pump 306 that may beoptionally included in fuel system 300 in some embodiments. Secondarytank 304 is fluidly coupled to main tank 200 so that the main tank maybe actively depressurized to allow refueling with low pressure liquidfuel. Control logic may be employed to determined, based on a firstthreshold, whether the pressure in main tank 200 may be reduced bypumping vapors from the main tank into secondary tank 304. The controllogic may also determine, based on a second threshold less than thefirst threshold, whether main tank 200 has been depressurized via vaporpumping to secondary tank 204 to an extent such that low pressurerefueling port 265 may be opened. FIG. 6 shows a flowchart illustratingan exemplary method 600 for refueling tank 200 via fuel system 300.

Turning now to FIG. 4, a flowchart illustrating an exemplary method 400for operating an engine and a fuel system is shown. Method 400 may beemployed for operating engine 10 of FIG. 1 and one or both of fuelsystems 172 and 300 of FIGS. 2 and 3, respectively, for example. Method400 begins at 410 where engine operating conditions such as engine oncondition (EOC), engine temperature, fuel system pressure, enginetorque, engine load, engine speed (RPM) and the like are measured and/orestimated. Method 400 continues at 420 where it determines if engineoperating conditions may increase generation of particulate emissions.At 424, method 400 determines if an engine temperature, T_(engine), isless than a threshold engine temperature, T_(engine,TH). IfT_(engine)<T_(engine,TH), liquid fuel droplets may fail to evaporatewhen striking metal surfaces of the engine and may thereby increasegeneration of particulate emissions. If T_(engine) is not less thanT_(engine,TH), method 400 continues at 428 where particulate generationis determined based on an engine speed and load. As an example, method400 may reference a look-up table of predetermined engine speed and loadto determine if particulate emissions may increase at the current enginespeed and load conditions. If method 400 determines that particulateemissions may not increase or are low at the current engine speed andload, method 400 continues at 430.

At 430, method 400 determines if a gaseous fuel bubble formation mayoccur in the fuel system. At 434, method 400 determines if an ambienttemperature, T_(ambient), is greater than a threshold ambienttemperature, T_(ambient,TH). T_(ambient) may also refer to a measured orestimated under-hood temperature or a fuel system temperature, asdescribed above. If T_(ambient)>T_(ambient,TH) gaseous fuel bubbles maybe generated during fuel delivery to the engine, and fuel deliveryreliability and robustness may be reduced. In one example, a fueldelivery volumetric flow rate may be lowered because of the expansion ofgaseous fuel bubbles in a liquid fuel line. In another example,formation of gaseous fuel bubbles may cause cavitation in a liquid fuelline or at a DI fuel pump, reducing fuel delivery reliability anddecreasing engine operability. In yet another example, fuel bubbles mayaffect fuel metering with fuel injectors, thus changing air/fuel ratioand degrading engine emissions. If T_(ambient) is not greater thanT_(ambient,TH), method 400 continues at 438 where it determines if afuel system pressure, P_(fuelsys), is less than a threshold fuel systempressure, P_(fuelsys,TH). If P_(fuelsys)<P_(fuelsys,TH), gaseous fuelbubbles may be generated during fuel delivery to the engine, and fueldelivery reliability and robustness may be reduced. As described above,P_(fuelsys) may be determined from one or more pressure sensorspositioned at fuel system 172 and/or 300. For example, with reference toFIG. 2, fuel system pressure may be measured by one or more of pressuresensor 211 at fuel tank 200, pressure sensor 238 at fuel separator 230,pressure sensor 223 in liquid fuel line 220, and a pressure sensor ineither of gaseous fuel rail 205 or liquid fuel rail 206.

If at 438, P_(fuelsys)<P_(fuelsys,TH), method 400 continues at 440 whereit determines if an engine load is less than a threshold engine load,Load_(TH). Injection of gaseous fuel (especially via port fuelinjection) may displace intake air in the engine cylinder or the engineintake air passage 146. As such, at high engine loads above Load_(TH),displacing engine intake air may reduce available engine torque anddecrease drivability and solubilized gaseous fuel may not be separatedfrom liquid fuel to enable injection of solubilized gaseous fuel inliquid fuel. If engine load is not less than Load_(TH), method 400continues at 460, where it opens the fuel separator bypass valve 226thereby directing the solubilized gaseous fuel and liquid fuel to bypassfuel line 228 and liquid fuel line 220. Furthermore, method 400 mayposition gaseous fuel switching valve 224 to fluidly connect fuel tank200 with gaseous fuel line 221. Next, at 464, the mixture of solubilizedgaseous fuel and liquid fuel may be injected via liquid fuel rail 206and liquid fuel injectors 166A and 166B to the engine. Because method400 determines that engine operating conditions are not conducive togeneration of particulate emissions at 420, and are not conducive togeneration of gaseous fuel bubbles at 430, the mixture of solubilizedgaseous fuel and liquid fuel may be injected to the engine whilemaintaining fuel delivery reliability and robustness. After 464, method400 ends.

If at 424 T_(engine)<T_(engine,TH), at 428 particulate generation mayincrease, at 434 T_(ambient)>T_(ambient,TH), at 438P_(fuelsys)<P_(fuelsys,TH), or at 440 engine load<Load_(TH), method 400proceeds to desolubilize and separate the gaseous fuel from the liquidfuel. At 470, method 400 closes the fuel separator bypass valve andpositions gaseous fuel switching valve 224 to connect fuel separator 230and gaseous fuel line 221. Next, at 472, method 400 directs thesolubilized gaseous fuel and the liquid fuel to the fuel separator 230,for example, via fuel lift pump 210. Alternately, the pressure in fueltank 210 may convey fuel from fuel tank 210. At 474, the gaseous fuel isdesolubilized and separated from the liquid fuel in fuel separator 230.As an example, a pressure differential across coalescing filter 234 maydesolubilize the gaseous fuel, whereby the gaseous fuel flows across thecoalescing filter 234 to expansion chamber 232, and out of fuelseparator 230 through gaseous fuel switching valve 224 to gaseous fuelline 221. As an example, a pressure in the expansion chamber may be lessthan a threshold pressure, for example less than 100 psi, in order toreadily desolubilize the gaseous fuel from the liquid fuel. Liquid fueldroplets may condense on coalescing filter 234 where they may coalesceand then drop back to the sump chamber 236 where most of the liquid fuelcollects before flowing out of fuel separator 230 to liquid fuel bypassline 228 and liquid fuel line 220. After the gaseous fuel and liquidfuel are separated in fuel separator 230, the gaseous fuel and theliquid fuel may be separately injected to the engine via the gaseousfuel injection system and the liquid fuel injection system at 476 and478, respectively. Method 400 ends following 476 and 478.

Turning now to FIG. 5, a flowchart illustrating an exemplary method 500for refueling a fuel system having a main fuel tank configured to storeboth a liquid fuel and a gaseous fuel. Method 500 may be used to refuelfuel tank 200 via fuel system 172 of FIG. 2, for example.

At 502 of method 500, it is determined whether refueling of the mainfuel tank is desired. A desire to refuel the main fuel tank may beindicated in various suitable manners. For example, a vehicle operatormay request refueling via a suitable interface. Alternatively oradditionally, opening of an outer door of a refueling port upstream ofthe main fuel tank may indicate a desire to refuel. As another example,vehicle proximity to a refueling station may indicate a desire torefuel. Vehicle proximity may be determined by various suitablemechanisms such as the GPS system described above or via location dataobtained in a different manner—for example, via direct communicationbetween the vehicle and the refueling station. If it is determined thatmain fuel tank refueling is not desired (NO), method 500 ends. If it isdetermined that main fuel tank refueling is desired (YES), method 500proceeds to 504.

At 504 of method 500, it is determined whether there is liquid fuelpresent in a surge tank (e.g., surge tank 270 of FIG. 2) of the fuelsystem. A liquid level sensor such as a float sensor (e.g., float sensor215 of FIG. 2) positioned in the surge tank may be used to determinewhether liquid fuel is present in the surge tank. In some embodiments,liquid fuel levels below a threshold may be treated as if no liquid fuelwas present in the surge tank. By detecting the presence of liquid fuelin the surge tank, pumping of air into the main fuel tank may beavoided, in turn avoiding the creation of a combustible fuel mixtureinside the main fuel tank. If it is determined that liquid fuel is notpresent in the surge tank (NO), method 500 returns to 502. If it isinstead determined that liquid fuel is present in the surge tank (YES),method 500 proceeds to 506.

At 506 of method 500, it is determined whether the pressure in the mainfuel tank exceeds a threshold pressure, or whether a level of liquidfuel in the main fuel tank exceeds a threshold level. Various suitablemechanisms may be employed to measure the pressure and/or liquid levelin the main fuel tank—for example, pressure sensor 211 (FIG. 2) andliquid level sensor 215 (FIG. 2) may be used to determine main fuel tankpressure and liquid level, respectively. If it is determined that thepressure in the main fuel tank exceeds the threshold pressure, or thatthe liquid level in the main fuel tank exceeds the threshold level(YES), method 500 ends, and refueling is disallowed. If it is insteaddetermined that neither the pressure in the main fuel tank exceeds thethreshold pressure, or that the liquid level in the main fuel tankexceeds the threshold level (NO), method 500 proceeds to 508.

At 508 of method 500, refueling is allowed and liquid fuel is pumpedfrom the surge tank to the main fuel tank. Fuel pump 285 of FIG. 2 maybe used to pump the liquid fuel, for example. Upon initiation of liquidfuel pumping from the surge tank to the main fuel tank at 508, method500 returns to 504 and conditionally to 506 such that the presence ofliquid fuel in the surge tank and main tank pressure and liquid levelmay be evaluated on an iterative basis to appropriately control liquidfuel pumping. As described above, if liquid fuel is not present in thesurge tank as determined at 504, or if the pressure or liquid level inthe main fuel tank exceed respective thresholds as determined at 506,method 500 ends and liquid fuel pumping from the surge tank to the mainfuel tank ends. Similarly, gaseous refueling of the main fuel tank maybe allowed if it is determined that the main fuel tank pressure andliquid level are below their respective thresholds as determined at 506.Allowance of gaseous refueling may include opening refueling lock 257 athigh pressure refueling port 255, for example.

Pumping of the liquid fuel from the surge tank to the main fuel tank at508 may include, at 509, determining a fuel volume with which to refuelthe main fuel tank based on a fuel inlet temperature and a main fueltank temperature. Fuel inlet temperature may be estimated or measuredvia a temperature sensor positioned at low pressure refueling port 265,while the main fuel tank temperature may be measured via temperaturesensor 212, for example. Here, the fuel volume may be determined basedon a difference between the fuel inlet temperature and the main fueltank temperature, and a maximum temperature that may be assumed by themain fuel tank during vehicle operation. Determination of the fuelvolume in this manner accounts for pressure increases in the main fueltank due to temperature increases. Without accounting for such pressureincreases, a fuel volume at a relatively cold temperature may besupplied to the main fuel tank and later exceed a maximum allowablepressure in the main fuel tank as the fuel volume heats up.Determination of the fuel volume may include accessing lookup tablesthat associate fuel inlet temperatures and main fuel tank temperatureswith fuel volumes specific to the types of liquid and gaseous fuelsbeing used, and the solubility of the gaseous fuels in the liquid fuels.

Next, at 510 of method 500, the fuel pump may be optionally run after athreshold duration has passed following determination that either themain fuel tank pressure or liquid level in the main tank have exceededrespective thresholds. Here, running the fuel pump after the thresholdduration may reduce evaporative emissions from the surge tank. Following510, method 500 ends.

Turning now to FIG. 6, a flowchart illustrating another exemplary method600 for refueling a fuel system having a main fuel tank configured tostore both a liquid fuel. Method 600 may be used to refuel fuel tank 200via fuel system 300 of FIG. 3, for example.

At 602 of method 600, it is determined whether refueling of a main fueltank is desired. As described above with reference to 602 of method 600,a desire to refuel the main tank may be indicated by a vehicle operatorrequest, opening of an outer door of a refueling port, location dataindicating proximity of the vehicle to a refueling station, via directcommunication between the vehicle and the refueling station, etc. If itis determined that main fuel tank refueling is not desired (NO), method600 ends. If it is determined that main fuel tank refueling is desired(YES), method 600 proceeds to 604.

At 604 of method 600, it is determined whether the pressure in the mainfuel tank is below a first threshold pressure. Pressure sensor 211 ofFIG. 2 may be used to measure the main fuel tank pressure, for example.If it is determined that the pressure in the main fuel tank is not belowthe first threshold pressure (NO), method 600 ends and refueling of themain tank is disallowed. If it is instead determined that the pressurein the main fuel tank is below the first threshold pressure (YES),method 600 proceeds to 606.

At 606 of method 600, it is determined whether the pressure in the mainfuel tank is below a second threshold pressure. In this example, thesecond threshold pressure is less than the first threshold pressure. Ifit is determined that the pressure in the main fuel tank is not belowthe second threshold pressure (NO), method 600 proceeds to 608 wherevapor is pumped from the main fuel tank to a secondary tank configuredto receive vapor from the main fuel tank. With reference to FIG. 3, thevapor may be pumped by pump 306 from fuel tank 200 to secondary tank304, for example. In this way, the main fuel tank may be refueled forinstances in which the main fuel tank pressure is between the first andsecond threshold pressures by pumping out vapor from the main fuel tankto the secondary tank. Method 600 may be adapted for embodiments inwhich the fuel system does not include a secondary tank; in thisexample, the main fuel tank may be depressurized by drawingsubstantially all of the gaseous fuel in the main fuel tank into theengine, or, in other examples, by venting gaseous fuel to the atmosphereor to another tank should one be available.

If it is instead determined that the pressure in the main fuel tank isbelow the second threshold pressure (YES), method 600 proceeds to 610where refueling of the main fuel tank with the liquid fuel is allowed.Refueling at 610 may include indicating to the vehicle operator thatrefueling is allowed and/or unlocking or opening an access door to arefueling port. Refueling with the gaseous fuel may also be allowed at610, which may include opening refueling lock 257 at high pressurerefueling port 255, for example.

Refueling the main fuel tank with the liquid fuel at 610 may include, at612, determining a fuel volume with which to refuel the main fuel tankbased on a fuel inlet temperature and a main fuel tank temperature. Thefuel volume may be determined in the manner described above withreference to 509 of method 500. As described above, determining of thefuel volume may account for pressure increases that may occur as thefuel heats up, and may include accessing lookup tables that associatefuel inlet temperatures and main fuel tank temperatures with fuelvolumes specific to the types of liquid and gaseous fuels being used,and the solubility of the gaseous fuels in the liquid fuels. Following612, method 600 ends.

Thus, as shown and described, methods 500 and 600 may be used to refuela fuel tank configured to store both liquid and gaseous fuels with theliquid fuel. While the approaches described herein are not limited toany particular type of gaseous or liquid fuel, usage of CNG (e.g.,methane) in combination with gasoline in particular may reduce engineknock while extending vehicular range and performance. Methods 400, 500,and 600 may be stored as machine-readible instructions in a storagemedium and executed by a logic subsystem—for example, the methods may bestored in RAM 112 of controller 12 and executed by CPU 106.

It will be appreciated that the configurations and methods disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

The invention claimed is:
 1. A system for supplying a liquid fuel and agaseous fuel to an internal combustion engine, the system comprising:one or more cylinders each associated with one or more fuel injectors; afuel tank storing both the liquid fuel and the gaseous fuel, the fueltank fluidly coupled to the one or more fuel injectors; and a controllerincluding instructions executable to: allow the liquid fuel to besupplied to the fuel tank only if a pressure in the fuel tank is lessthan a threshold pressure; and disallow the liquid fuel to be suppliedto the fuel tank if the pressure in the fuel tank is greater than thethreshold pressure.
 2. The system of claim 1, wherein allowing theliquid fuel to be supplied to the fuel tank includes pumping the liquidfuel from a surge tank to the fuel tank via a fuel pump, the surge tankconfigured to store the liquid fuel and fluidly coupled to the fuel tankupstream of the fuel tank.
 3. The system of claim 2, wherein theinstructions are further executable to run the fuel pump after athreshold duration to reduce evaporative emissions from the surge tank.4. The system of claim 1, wherein supply of the liquid fuel to the fueltank is disallowed if a liquid level in the fuel tank is greater than athreshold level.
 5. The system of claim 1, wherein the liquid fuel isreceived at a low pressure refueling port upstream of the fuel tank, thegaseous fuel is received at a high pressure refueling port upstream ofthe fuel tank, and the gaseous fuel is at least partially solubilized inthe liquid fuel in the fuel tank.
 6. The system of claim 1, wherein theinstructions are further executable to depressurize the fuel tank bypumping vapor from the fuel tank to a secondary tank before allowing theliquid fuel to be supplied to the fuel tank.
 7. A method, comprising:storing liquid gasoline having methane soluble therein; separating themethane from the liquid gasoline and delivering each separately to anengine cylinder; and enabling refueling of said liquid gasolinedepending on an operating condition.
 8. The method of claim 7, whereinthe liquid gasoline is stored in a fuel tank, and wherein the operatingcondition is one of a pressure and a liquid level in the fuel tank.